It is but you can't dock wages with a legitimate WTR because the employees are working to exact specifications of their contract of employment. In theory, therefore, they aren't in breach of the contract and aren't open to docking of wages or any form of action against them.
The only avenue would be if there is a specific link in the contract and pay to certain performance criteria and standards, say operating standards. These would set the specifics on the duties of certain roles and could be seen (depending on the specifics of the contracts) as part of their contract of employment.
However, if these don't exist and the duties have never been formalised, there is nothing illegal with the WTR and nothing that is a breach of contract.